1. Field of the Invention
The present invention relates to an optical amplifier using a rare earth element doped optical fiber that is to be excited with a semiconductor laser.
2. Description of the Related Art
Recently, an optical amplifier that amplifies light at 1.5 .mu.m using silica fibers doped with Er (erbium) and other rare earth elements are being commercialized.
FIG. 4 is a schematic view showing the optical amplifiers. As shown in the drawing, the optical amplifier basically includes a rare earth element doped optical fiber 1 serving as actual light amplifying element, a light source 2 emitting exciting light injected into the optical fiber 1, and an optical fiber 3 guiding light to be amplified with the optical fiber 1. The exciting light source 2 has not only a semiconductor laser device 21 which is a light source in the true sense of the term but also a drive circuit 22 including a DC power source for supplying drive power to the laser device, an automatic output control circuit, etc., as well as an optical fiber 20 for coupling the output light from the semiconductor laser device 21 to the rare earth element doped optical fiber 1. The optical fiber 1 has a filter 4 inserted at the output and for rejecting the exciting light component from the output light.
The optical amplifier composed in the way described just above can employ a semiconductor laser operating at a wavelength of 1.48 .mu.m or 0.98 .mu.m as a source of exciting light. On the other hand, studies are being made of optical amplifiers that amplify light at 1.3 .mu.m replacing the Er doped optical fiber by a Pr (praseodymium) doped fluoride optical fiber.
Semiconductor lasers used as a source of exciting light in the optical amplifier of the type described above share the common problem that their operation is instabilized by light traveling in the return path. Referring to FIG. 4, if light returning from the rare earth element doped optical fiber 1 towards the light source is reinjected into the semiconductor laser 21, so-called "return light noise" will increase to make the operation of the optical amplifier instable.
In order to solve this problem, it has been proposed with an optical amplifier using a semiconductor laser operating at a wavelength of 1.48 .mu.m that the return light is blocked with an optical isolator inserted between the optical fiber and the semiconductor laser. This proposal is actually implemented.
FIG. 5 is a schematic view showing the general layout of an optical amplifier employing an optical isolator. In FIG. 5, the components which are the same as those shown in FIG. 4 are identified by same numerals.
In FIG. 5, in addition to the optical amplifier shown in FIG. 4, an optical isolator 5 is inserted between the semiconductor laser 21 and the optical fiber 20. Once the light emitting from the semiconductor laser 21 passes through the optical isolator 5, no light will return to the semiconductor laser 21 because of the blocking action of the isolator 5.
As described above, it is known that the operation of an optical amplifier can be stabilized by using an optical isolator to block the light traveling in the return path to the semiconductor laser. However, the optical isolator that has been developed to date for the purpose of blocking the return light is only adaptive to operations at a wavelength of 1.48 .mu.m, and the layout shown in FIG. 5 cannot be realized for an optical amplifier that uses a semiconductor laser operating at a wavelength of 0.98 .mu.m.